Chronic wounds can be caused by a variety of events, including surgery, prolonged bed rest, and traumatic injuries. Partial thickness wounds can include second degree burns, abrasions, and skin graft donor sites. Healing of these wounds can be problematic, especially in cases of diabetes mellitus or chronic immune disorders. Full thickness wounds have no skin remaining, and can be the result of trauma, diabetes (e.g., leg ulcers), and venous stasis disease, which can cause full thickness ulcers of the lower extremities. Full thickness wounds tend to heal very slowly. Proper wound care technique, including the use of wound dressings, is extremely important to successful chronic wound management. Chronic wounds affect an estimated four million people a year, resulting in health care costs in the billions of dollars. "Treatment of Skin Ulcers with Cultivated Epidermal Allografts," T. Phillips, O. Kehinde, and H. Green, J. Am. Acad. Dermatol., V. 21, pp. 191-199 (1989).
The wound healing process involves a complex series of biological interactions at the cellular level, which can be grouped into three phases: hemostasis and inflammation, granulation tissue formation and reepithelization; and remodeling. "Cutaneous Tissue Repair: Basic Biological Considerations," R. A. F. Clark, J. Am. Acad. Dermatol., Vol. 13, pp. 701-725 (1985). Keratinocytes (epidermal cells that manufacture and contain keratin) migrate from wound edges to cover the wound. Growth factors such as transforming growth factor-.beta. (TGF-.beta.) play a critical role in stimulating the migration process. The migration occurs optimally under the cover of a moist layer. Keratins have been found to be necessary for reepithelization. Specifically, keratin types K5 and K14 have been found in the lower, generating, epidermal cells, and types K1 and K10 have been found in the upper, differentiated cells. Wound Healing: Biochemical and Clinical Aspects, I. K. Cohen, R. F. Diegleman, and W. J. Lindblad, eds., W. W. Saunders Company, 1992. Keratin types K6 and K10 are believed to be present in healing wounds, but not in normal skin. Keratins are major structural proteins of all epithelial cell types and appear to play a major role in wound healing.
An optimum wound dressing would protect the injured tissue, maintain a moist environment, be water permeable, maintain microbial control, deliver healing agents to the wound site, be easy to apply, not require frequent changes, and be non-toxic and non-antigenic. Although not ideal for chronic wounds, several wound dressings are currently on the market, including occlusive dressings, non-adherent dressings, absorbent dressings, and dressings in the form of sheets, foams, powders, and gels. Wound Management and Dressing, S. Thomas, The Pharmaceutical Press, London, 1990.
Attempts have been made to provide improved dressings that would assist in the wound healing process using biological materials such as growth factors. To date, these biologicals have proven very costly and have shown minimal clinical relevance in accelerating the chronic wound healing process. In cases of severe full thickness wounds, autografts (skin grafts from the patient's body) are often used. Although the graft is non-antigenic, it must be harvested from a donor site on the patient's body, creating an additional wound. In addition, availability of autologous tissue may not be adequate. Allografts (skin grafts from donors other than the patient) are also used when donor sites are not an option. Allografts essentially provide a "wound dressing" that provides a moist, water-permeable layer, but is rejected by the patient usually within two weeks and does not become part of the new epidermis.
What would be desirable, and has not heretofore been provided, is a wound dressing that protects the injured tissue, maintains a moist environment, is water permeable, is easy to apply, does not require frequent changes, is non-toxic and non-antigenic, and most important, delivers effective healing agents to the wound site.
Film materials compatible with living tissue are useful for a number of applications including tissue engineering scaffolding, diffusion membranes, coatings for implantable devices, and cell encapsulants. Bulk keratin materials compatible with living tissue are useful for a number of applications including open cell tissue engineering scaffolding and bulk, cross-linked biomaterials. Tissue engineering is a rapidly growing field encompassing a number of technologies aimed at replacing or restoring tissue and organ function. The consistent success of a tissue-engineered implant rests on the invention of a biocompatible, mitogenic material that can successfully support cell growth and differentiation and integrate into existing tissue. Such a scaffolding material could greatly advance the state of the tissue engineering technologies and result in a wide array of tissue engineered implants containing cellular components, such as osteoblasts, chondrocytes, keratinocytes, and hepatocytes, to restore or replace bone, cartilage, skin, and liver tissue respectively.
Diffusion membranes are commonly formed of synthetic polymeric materials, rather than biologically-derived materials. Diffusion membranes derived from biological materials have the advantage of enhanced biocompatibility. In particular, non-antigenic diffusion membranes are compatible with implantation in the human body and would provide great advantages in controlled drug release applications.
Implantable devices, such as pacemakers, stents, orthopedic implants, urological implants, dental implants, breast implants, and implants for maxillofacial reconstruction are currently encased in, or made of, materials including titanium, silicone, stainless steel, hydroxyapatite, and polyethylene, or encapsulated in materials such as silicone or polyurethane. These metals, ceramics, and synthetic polymers have disadvantages related to biocompatibility and antigenicity which can lead to problems related to the long term use of these devices. A coating material derived from biological materials and having non-antigenic and mitogenic properties would provide a device the advantage of long term biocompatibility in vivo and potentially extend the useful lifetime of an implant while decreasing the risk of an allergic or negative immune response from the host.
Cell encapsulants such as Chitin/Alginate and bovine-derived collagen are used to encapsulate mammalian cells for applications such as tissue engineering/organ regeneration and bacteria for cloning applications. A non-antigenic, non bioresorbable cell encapsulant material would have the advantages of providing the cell with a mitogen and increasing the chances for the cell to accomplish its tissue engineering function.
A bulk, cross-linked implantable biomaterial that was non-antigenic and possessed the appropriate mechanical properties could be used for maxillofacial restoration, for example, for both soft and hard tissue replacement. Such a bulk material could also be used for orthopedic applications as a bone filler and for cartilage regeneration. A bulk material capable of being implanted could also be used for neurological applications, such as for nerve regeneration guides.
Keratin, often derived from vertebrate hair, has been processed into various forms. Commonly assigned U.S. Pat. No. 5,358,935 discloses mechanically processing human hair into a keratinous powder. The hair is bleached, rinsed, dried, chopped, homogenized, ultrasonicated, and removed from solvent, leaving a keratin powder. In U.S. Pat. No. 5,047,249, Rothman discusses activating keratin with a reducing agent and applying the activated keratin to a wound. Rothman believes the activated keratin thiol groups will react with thiol groups in the wound tissue and form a disulfide bond, allowing the keratin to adhere to and protect the wound.
Keratin derived materials are believed to be non-antigenic, particularly when derived from a patient's own keratin. A film formed from keratin based material would be desirable. A keratin film able to be used for tissue-engineering scaffolds, diffusion membranes, implantable device coatings, and cell encapsulants would be very useful. A solid keratin bulk material would also have great utility. In addition, a non-antigenic, mitogenic open cell keratin scaffold would prove highly beneficial for use as a tissue engineered scaffold to support, nourish, and stimulate cell growth preceding and following implantation.